REFRACTORY METAL CONTACT FOR A POWER DEVICE

Abstract: Using a refractory metal for metallization in an integrated circuit, the same minimum size transistor structure can be used in the same manufacturing process to build linear and logic circuits, without requiring an increased spacing between the drain contact and the channel when the transistor is used in a high current application. Using a refractory metal, reliability problems, such as metal/silicon interdiffusion and electromigration, relating to high dissipation in the drain regions are avoided.

Description: Refractory Metal Contact for a Power Device Background of the Invention 1. Field of the Invention The present invention relates to the physical design of transistors; and, in particular, the present invention relates to the design of high current MOS transistors for avoidance of both silicon-metal interdiffusion and electromigration. 2. Discussion of the Related Art In a conducting MOS transistor, the highest dissipation region is located at the drift region (i.e. the depletion region of the drain region) at the junction between the drain region and the channel region. In particular, the highest power dissipation occurs at the drift region because of the substantial voltage drop accross this junction. While high dissipation is seldom a concern in logic circuits, which have few devices conducting static currents (i.e. currents having frequencies of less than 20 Hz) , high dissipation due to static currents is a serious concern in linear power amplifier circuits, which typically conduct significant amount of static currents in the output stage transistors. For example, a static current of up to 600mA is common in an audio amplifier application under normal operating conditions (i.e. driving an 8-ohm load); a higher current can result under a fault condition. In MOS circuits using an aluminum-based metallization (e.g. aluminum or aluminum-copper alloys) , high dissipation leads to a number of reliability problems including aluminum diffusion into silicon and electromigration. Diffusion of aluminum into silicon results from the high temperature caused by high dissipation near the drain contact. Since the interdiffusion coefficient between aluminum and silicon is relatively high and temperature sensitive 1 , a higher temperature leads to excessive diffusion of aluminum into the drain region, causing performance degradation over time. Electromigration is a mass transport phenomenon which is caused by high average current over time. Electromigration is more severe in a metal of lower atomic weight than one with higher atomic weight. In a conducting MOS transistor, electromigration can create an open circuit at the drain contact. In, an integrated circuit, a large power transistor is normally provided by connecting a large number of minimum dimension transistor structures in parallel. In the prior art, to avoid the temperature-induced effects on the drain contact resulting from the high power dissipation, the spacing in each minimum dimension transistor structure between the drain contact and the channel is provided to be greater than the spacing normally used for a minimum dimension transistor of a logic circuit using the same manufacturing process. However, in a linear power amplifier integrated circuit, such as an audio output amplifier, output power transistors occupy the majority of the die area, so that an increased spacing in each minimum dimension transistor structure between the drain contact and the channel region results in a severe area penalty. Such area penalty translates into a correspondingly undesirable economic cost per die. Summary of the Invention In accordance with the present invention, a manufacturing process using a refractory metal for 1 A discussion on the diffusion of aluminum in silicon can be found in the article "Diffusion of Al in Si" in Properties of Silicon , published by INSPEC, the Institute of Electrical Engineers, pp. 339-341. metallization allows the same minimum drain contact- channel spacing to be used for both linear and logic applications. The present invention provides significant area savings in a power amplifier integrated circuit application. Further, the power transistors created by the manufacturing process of the present invention are relative free of metal/silicon interdiffusion and electromigration related reliability problems. The present invention is better understood upon consideration of the detailed description below and the accompanying drawings. Brief Description of the Drawings Figure 1 compares the drain contact to channel spacing requirements between a manufacturing process using a refractory metal, in accordance with the present invention, and a manufacturing process using a conventional aluminum-based metal. Figure 2 shows a layout 200 of a power transistor implemented in accordance with the present invention. Detailed Description of the Preferred Embodiments In accordance with the present invention, one level of metallization in an integrated circuit manufacturing process is provided by a refractory metal, such as tantalum, tungsten, titanium, molybdenum, palladium, platinum, or their alloys. The metallization can be provided on the integrated circuit using any conventional metallization process, such as sputtering or chemical vapor deposition. Figure 1 shows schematically one embodiment of the present invention in an MOS transistor structure 101. MOS transistor structure 101 is a transistor structure of the minimum dimensions allowed under a particular manufacturing process. As shown in Figure 1, transistor structure 101 includes a common source region 110, drain regions 111 and 113, and channel regions 112 and 114. Drain regions 111 and 113, and source region 110 are contacted by metal lines 119, 117 and 118 respectively. Contacts to drain regions 111 and 113, and source region 110 are created by vias through one or more layers of insulative thin films (not shown in Figure 1) . Channel regions 112 and 114 are each controlled by a voltage at gate terminals provided by polysilicon lines 115 and 116. Thus, with proper isolation, transistor structure 101 can be used to form two minimum size MOS transistors. Alternatively, if the gate terminals provided by polysilicon lines 115 and 116 are connected in common, a transistor twice the minimum size can be formed. Further, transistor structure 101 can be duplicated in large number and connected in common to form a power transistor of any size. In accordance with the present invention, metal lines 117, 118 and 119 of transistor structure 101 are formed by a refractory material, such as tungsten. In this embodiment, which is part of an audio amplifier integrated circuit, metal lines 117 and 119 are shorted to each other and polysilicon lines 115 and 116 are shorted to each other, to form part of a larger power transistor. In one implementation, a self-aligned manufacturing process uses the widths of polysilicon lines 115 and 116 to define the lengths of channel regions 112 and 114. In that implementation, the channel lengths are each drawn 1.0 microns, which is optically shrunk to define a channel region of 0.8 microns on silicon. In that implementation, drain regions 111 and 113 are drawn 12.8 microns apart, measured from the drain contact to drain contact. Of significance in this implementation is the spacing, indicated by "a" in Figure 1, between the contact region 103 of drain region 111 and the closest edge of polysilicon line 115. Spacing "a" is drawn to be 1.0 micron wide, which is the same minimum spacing required between a drain contact and a polysilicon line for logic circuits designed under the same manufacturing process. A transistor structure 102 is also shown in Figure 1 for comparison purpose. In contrast to transistor structure 101's spacing "a", the corresponding spacing "b" in transistor structure 102 would have been four microns under a hypothetical prior art process using aluminum for metallization. Transistor structure 102 would have substantially the same dimensions for its common source region 120 and channel regions 122 and 124 as the corresponding common source region 110 and channel regions 112 and 114 in transistor structure 101. However, since transistor structure 102's spacing "b" is required to be larger than transistor structure 101 , s corresponding spacing "a" due to the high temperature consideration of the prior art, the distance between the drain contacts 104 and 130 in transistor structure 102 would have been drawn 18.8 microns apart. The corresponding drain contact to drain contact distance, in the implementation of the present invention discussed above, is drawn 12.8 microns apart. Thus, the present invention provides an area saving of approximately 50%. In the implementation of transistor structure 101 discussed above, an audio amplifier with a target dissipation of 5 watts operating with a 5 volt power supply is driven by an output stage in bridge configuration. The output stage includes a p-channel power transistor and an n-channel power transistor each formed by a large number of transistor structures each similar to transistor structure 101 discussed above, in accordance with the present invention. One layout 200 of such an implementation is shown in Figure 2. Layout 200 can be used for either a p-channel transistor or an n- channel transistor, although the difference in on- resistance between a p-channel transistor and an n-channel transistor requires that the p-channel transistor be sized approximately three times the size of a corresponding n- channel transistor to provide the same current capability. In layout 202, pairs of polysilicon strips, such as the pair of strips 201a and 201b, are provided at regular intervals and connected in common by a metal line 202. Strips 201a and 201b correspond to the polysilicon lines 115 and 116 of Figure 1. A first set of regularly spaced metal lines, such as metal lines 204 and 207, are each provided to contact a row of underlying drain regions. A second set of regularly spaced metal lines, such as metal lines 203 and 208, are each provided to contact two parallel rows of source regions. For example, in metal line 208, contacts 205a and 205b each contact a source region in a different one the two parallel rows of source regions underlying metal line 208. The contact 206 between contacts 205a and 205b is a contact to the underlying well, for the purpose of providing a substrate bias voltage. Because of the high' melting point of the refractory metal, in a power transistor of the present invention, the phenomenon of metal/silicon interdiffusion is insignificant even though the same drain contact to polysilicon minimum spacing is used in a linear circuit as in a logic circuit. Further, electromigration in refractory metal is also considerably less than that of aluminum. The above detailed description is provided to illustrate the specific embodiments of the present invention and is not to be taken in a limiting sense. Numerous variations and modifications are possible within the scope of the present invention. The present invention is defined by the following claims.

Claims: Claims I claim: 1. A method for manufacturing an integrated circuit in a given manufacturing process, said manufacturing process defining a minimum distance between a contact in a drain region and a channel region, said method comprising the steps of: defining a transistor structure on a semiconductor substrate, said transistor structure defining a transistor having a drain region and a channel region for conducting a static current during operation; defining a contact to said drain region, said contact being spaced from said channel region by a distance substantially equal to said minimum distance; and manufacturing said transistor structure using said manufacturing process, providing as metallization to said contact a refractory metal. 2. A method as in Claim 1, wherein said method uses a refractory metal selected from the group consisting of tungsten, tantalum, molybdenum, palladium, platinum, and alloys of said group. 3. An integrated circuit fabricated under a manufacturing process requiring a minimum distance between a contact in a drain region and a channel region, said integrated circuit comprises: a transistor structure on a semiconductor substrate, said transistor structure defining a transistor having a drain region and a channel region for conducting a static current during operation; a contact to said drain region, said contact being spaced from said channel region by a distance substantially equal to said minimum distance; and a metal trace to said contact comprising a refractory metal. 4. An integrated circuit as in Claim 3, wherein said refractory metal is selected from the group consisting of tungsten, tantalum, molybdenum, palladium, platinum, and alloys of said group.

Inventors: JOHNSON NICKY M, JOHNSON, NICKY, M.
Applicants: NAT SEMICONDUCTOR CORP, NATIONAL SEMICONDUCTOR CORPORATION
Application year: 1994
Main classification: H01L29
IPC codes: H01L29, H03F1, H03F3